1. Field of the Invention
The present invention relates to a switched reluctance motor(SRM), and more particularly to a driving control apparatus for the SRM for preventing an excessive driving current from flowing into the SRM.
2. Description of the Prior Art
In general, a switched reluctance motor(SRM) includes, as shown in FIG. 1, a stator 2 and a rotor 4, wherein the stator 2 has three pairs of poles +A and -A, +B and -B, and +C and -C(or, salient pole C) on which A phase coil 6, B phase coil 8, and C phase coil 10 are wound, respectively.
A driver for the SRM thus constructed consists of, as shown in FIG. 2, a smoothing capacitor C for producing a direct current voltage, a plurality of switching elements Q1 to Q6 for applying the voltage to the respective phase coils 6, 8, and 10, 6 diodes D1 to D6 for feedback of a back electromotive force created at the time of turning-off of the respective switching elements Q1 to Q6 after applying the voltage to each said phase coil.
The switching operation of the A-phase switching elements Q1 and Q2 may be controlled by a signal produced from a gate signal generating unit. This unit includes, as shown in FIG. 3, an oscillator 12 which outputs a Pulse Width Modulated(PWM) signal, an AND logic gate 16 which produces a logic product of said produced PWM signal and a position information signal available from an A phase position sensor 14 when the rotor 4 faces with A phase pole of the stator 2, wherein the signal from the AND logic gate 16 is applied to an upper A phase switching element Q1, and wherein the position information signal from the A phase position sensor 14 is applied to a lower A phase switching element Q2.
An application of a power to the A phase of the SRM may be made by giving operational signals to the gate of A phase-related switching elements Q1, Q2. The activated switching elements Q1, Q2 allow electric current to flow through the A phase coil of the stator 2, resulting in magnetization of the poles +A and -A of the stator. Such magnetized poles pull the rotor 4 positioned near the A phase pole.
B and C phase coils 8 and 10 also operate as in A phase-related action, similarly, wherein the order of A, B and C phase for the magnetization of the stator 2 is maintained, by which the motor can continue to rotate.
However, reliability of the SRM conventionally used cannot be guaranteed when abnormal operation and surge current occur to the driver, to further render the efficiency of the motor to become lower. Moreover, there is a problem in that loud noise from the motor is generated due to current ripple, or torque ripple phenomenon at an initial start-up at each phase.
FIG. 4 shows a schematic block diagram of a driver for a variable reluctance motor, or SRM disclosed in U.S. Pat. No. 5,225,758, where the driver controls the motor in such a manner that an average voltage linearly varies which is to be supplied to the coil for a current deviation or current command. For this, the driver has an absolute value circuit 20, a PWM circuit 21, a polarity determining circuit 12, a signal selection circuit 23, first and second drivers 24a, 24b, an On/Off signal generator 30, and a driving circuit 40.
The absolute value circuit 20 receives both a current command from a host computer and a current deviation for a current in reality, compares them with a carrier signal, produces an output representing an absolute value for the current deviation, and outputs it to the PWM circuit 21, wherein the PWM circuit 21 compares the absolute value for the current deviation with the carrier signal to obtain a PWM signal, and then outputs it to the signal selection circuit 23.
In case the current deviation is above 0 volt, the polarity determining circuit 12 outputs a low level signal to turn on a first transistor Q1 in the driving circuit 40. During high level of the output signal from the On/Off signal generator 30, a second transistor Q2 is turned on/off depending upon the PWM signal output from the PWM circuit 21.
In case the current deviation is below 0 volt, the second transistor Q2 is turned off by the signal selection circuit 23, and the first transistor Q1 is turned on/off by the PWM signal.
As a result, the average coil voltage linearly varies with respect to the current deviation, or the current command, by which the control for the motor can readily be made.
The above driver for the SRM determines a duty ratio for the PWM in response to the current deviation or the current command, and selects and operates switching element for applying the PWM based upon the polarity of the current deviation, so as to result in a linear variation of the average coil voltage with respect to the current deviation or the current command.